I. Field of the Invention
The present invention relates to PIN diode attenuators, and more particularly to using a Zener diode array to linearize the response of a PIN diode attenuator.
II. Related Art
Automatic gain control circuits (AGC) generally are used to maintain a predetermined signal level at an output despite variations in the signal level at an input. Such circuits are used in a wide variety of electronic devices including radio and television receivers and other communication systems. In transmitting systems, such circuits are sometimes referred to as automatic level control (ALC) circuits. One example of an AGC circuit includes the use of an attenuator in a signal receiving circuit. In such systems, the attenuator operates to provide a relatively constant radio frequency (RF) signal output for a varying RF signal input by varying an impedance between the RF input and RF output.
A PIN diode attenuator is a commonly used circuit for providing such attenuation. A PIN diode attenuator is a device that provides a predetermined value of attenuation in a transmission line, in response to a precise value of bias. IEEE Standard Dictionary of Electrical and Electronics Terms, 4.sup.th Edition, IEEE, New York, N.Y. (1988). In other words, a PIN diode attenuator provides variable attenuation for a signal passing through it. A bias signal dictates how much attenuation is provided by the PIN diode attenuator. A PIN diode attenuator circuit generally includes multiple PIN diodes. A PIN diode is a diode with a large intrinsic region located between the p-n junction of the diode. This provides for a sharp knee in the PIN diode i-v characteristic curve.
During normal operation, a PIN diode attenuator is placed in the signal path of a signal receiving or transmitting circuit, and is used to provide variable attenuation for the level of the signal passing through it. The response of a conventional PIN diode attenuator is non-linear. The slope of a PIN diode attenuator response curve is related to the response time of the circuit. When the slope of the response curve varies over its length, the response times of the circuit at different points along the curve are different. The slope of the response of a PIN diode attenuator varies all along the curve. It is desired for the response curve of a PIN diode attenuator to be linear. What is needed is to achieve a range where the rate of change of attenuation (dB) with control voltage variation is a constant. This would provide for a more uniform response time for the attenuator within this range.
One way to create a linear response for a PIN diode attenuator is to combine it with another circuit, such that when their responses are combined, a linearized response results. Such a linearization circuit may be inserted into the feedback path of the signal transmitting or receiving circuit. One possible linearization circuit uses an analog-to-digital (A/D) converter to sample the feedback signal. The sampled value is used to access a ROM with a look-up table. The ROM maps out all of the feedback signal levels to the desired attenuation level. The ROM output is input to a digital-to-analog (D/A) converter, and fed back as a bias to the variable gain PIN diode attenuator. The A/D-ROM-D/A combination can be configured to create a response that, when combined with the non-linear response of a PIN diode attenuator, linearizes the response of the PIN diode attenuator. This approach, however, is complicated and requires a substantial number of circuit components or digital ICs to implement. It is desired to create a less complicated and less expensive linearization circuit.
Another approach for linearizing the response of a PIN diode attenuator uses an analog circuit inserted into the feedback path to emulate the desired linearization circuit response. One such approach uses an array of Zener diodes of incremental Zener voltage values, with as many Zener diodes as required by the particular application. The cathodes of the Zener diodes are coupled in common to the feedback signal. The anodes of the Zener diodes are coupled to respective resistors. The resistors are then coupled in common to the input of an amplifier. The output of the amplifier is coupled to the PIN diode attenuator bias point. The voltage values for the Zener diodes generally are chosen such that they increase in steps. For example, three Zener diodes in such a conventional Zener circuit may have voltage values of 3.3 V, 6.2 V, and 9.1 V, respectively. As the voltage level of the feedback signal increases, one diode turns on after the other. When a Zener diode turns on, its adjoining resistor affects the gain of the circuit. By calibrating all of these resistor values, the desired response for the circuit may be approximated.
This particular Zener diode circuit approach has limitations. Such a Zener diode circuit does not work consistently across variations in temperature. This is due primarily to differences in the temperature coefficients of different Zener diodes, in particular, zener diodes with different voltages. A plot of temperature coefficients shows that for Zener diodes with a voltage value of around 5.1V, the temperature coefficient is roughly zero. Hence, a 5.1 V Zener won't change its voltage much with variations in temperature. Zener diodes of voltages below 5.1 V, however, have a negative temperature coefficient. This means that as temperature increases, a Zener diode with a voltage value of less than 5.1 V will correspondingly decrease in voltage value. As temperature decreases, such a Zener diode will correspondingly increase in voltage value. Zener diodes with a voltage value greater than 5.1 V have a positive temperature coefficient. These Zener diodes will increase in voltage when temperature increases, and will decrease in voltage when temperature decreases. Thus, if Zener diodes with voltage values such as 3.3 V or 9.1 V are used, the voltage drops across the Zener diodes will vary with temperature. Such voltage variations will cause the slope of the response curve of the overall circuit to change with temperature variations. For instance, at ambient temperature, the Zener diode will produce a linearized response for the overall circuit. The slope of this linearized response, however, will become more or less steep with variations in the environmental temperature due to the temperature coefficients of the various Zener diodes involved. Changes in the slope of the response curve will adversely affect the gain of the overall circuit. Such variations in circuit performance are undesirable. What is needed is a feedback circuit that will provide a linearized response that is stable over a wide range of temperatures.